This invention relates to the field of technology in the apparatus and the method for reading radiation images that are typically used in diagnostic radiology. More particularly, the invention relates to an apparatus and a method for reading radiation images that are adapted for detection of the linearity in image reading by a simple procedure.
A system for diagnostic radiology using a stimulable phosphor has been commercialized as exemplified by CR (Computed Radiography) such as FCR (Fuji Computed Radiography manufactured by Fuji Photo Film Co., Ltd.). When illuminated with a radiation (e.g. X-rays, xcex1-rays, xcex2-rays, xcex3-rays, electron beams or ultraviolet rays), a stimulable phosphor accumulates a portion of the radiation energy and, upon subsequent illumination with exciting light such as visible light, the phosphor produces stimulated light emission in accordance with the accumulated energy.
In a system for diagnostic radiology using the stimulable phosphor, the radiation image of a subject such as the human body is radiographed and recorded in a sheet having a stimulable phosphor layer (hereunder referred to as a xe2x80x9cphosphor sheetxe2x80x9d), which is scanned two-dimensionally with exciting light to emit stimulated light which, in turn, is read photoelectrically to produce a picture signal on the basis of which the radiation image of the subject is output as a visible image on a photographic material such as a radiographic film or a display device such as a CRT (see, for example, U.S. Pat. No. 4,258,264 and Unexamined Published Japanese Patent Application (kokai) No. 11395/1981).
The system using a stimulable phosphor has many advantages and its current use in the area of medical diagnosis is well anticipated to extend to non-medical fields such as the detection of flaws or cracks in industrial products. For example, the transmitted radiation image of round bars and steel pipes may be reproduced in the manner just described above. Since a damaged area absorbs less radiation than the sound area, it comes out darker than the surrounding area of the reproduced image, thus enabling the detection of the flaw or crack that cannot be seen from the outside.
To perform correct diagnosis and detection by means of the radiation imaging system, the subject radiographed and recorded in the phosphor sheet must be reproduced to give the correct image. In the radiation imaging system, the signal obtained by reading the radiation image usually receives processing such as A/D conversion and log conversion to produce digital image data for the radiographed and recorded radiation image. To reproduce the correct image, the image reading apparatus must maintain the intended linearity. In other words, the dose of radiation to which the radiation recording medium such as the phosphor sheet has been exposed during radiation imaging (which is hereunder sometimes referred to as the xe2x80x9cexposed dosexe2x80x9d) must have a linear (1:1) correspondence with the image data obtained by the image reading process.
The linearity of the image reading apparatus is one of the important characteristics for ensuring that the image that has been read is correctly reproduced as a visible image. If the linearity changes, inconveniences occur as exemplified by the change in the contrast of the reproduced image and this leads to an error in diagnosis by a system for diagnostic radiology.
It is therefore a first object of the present invention to provide a radiation image reading apparatus that is suitable for use with a radiation imaging system typically using a phosphor sheet, that is capable of checking for linearity by a simple procedure, with an optional capability of linearity adjustment in accordance with the result of checking, and which enables consistent outputting of diagnostic images with appropriate contrast.
A second object of the present invention is to provide a radiation image reading method that is executed simply and easily in the above described radiation image reading apparatus.
This first object of the invention can be attained by a radiation image reading apparatus comprising a reading device of a radiation image recorded in a radiation recording medium, a dose detecting device which detects a dose of radiation applied to record the radiation image in the radiation recording medium and a computing device by which linearity in radiation image reading is calculated using a plurality of radiation images with varying doses of radiation that have been read by the reading device, as well as the doses of radiation applied to record the respective radiation images and which have been detected by the dose detecting device.
Preferably, the radiation recording medium has the plurality of radiation images with varying the doses of radiation recorded therein by split recording. In another preferred embodiment, the radiation recording medium has the plurality of radiation images with varying the doses of radiation recorded therein using at least one filter having a known radiation transmittance. Preferably, the at least one filter is a radiation filter in a form of a step wedge that transmits a radiation. Preferably, the radiation filter in the form of the step wedge transmits the radiation by varying degrees of 50%, 20% and 5%.
In yet another preferred embodiment, the apparatus further comprises a correction device that corrects for the linearity in accordance with a result of the linearity calculation by the computing device.
Preferably, a difference between the dose of radiation in the plurality of the radiation images is in a range of from 1:3 to 1:1000 in terms of a ratio. Preferably, when image data of the radiation image is antilogarithm data reverted to a linear state, the linearity is expressed as a ratio in the image data divided a ratio in exposed dose and if the calculated linearity is outside a range of from 0.8 to 1.2 the correction device corrects for the linearity.
Preferably, when image data Q0 and Q1 of the radiation images are log converted density data having a 4-digit dynamic range assigned to 10 bits and are read from the radiation images recorded by applying the doses X0 and X1 of radiation, respectively, the linearity is expressed by a following equation and if the calculated linearity is outside a range of from 0.8 to 1.2 the correction device corrects for the linearity.
the linearity={(Q1xe2x88x92Q0)/256}/log(X1/X0)
Preferably, the dose detecting device is a dosimeter for measuring the dose of radiation applied to record the radiation image. Preferably, the radiation recording medium is a stimulable phosphor sheet.
The second object of the invention can be attained by a radiation image reading method comprising the steps of detecting doses of radiation applied to record the respective radiation images in at least one radiation recording medium by varying the doses of radiation, reading the radiation images recorded in the at least one radiation recording medium and calculating linearity in radiation image reading using a plurality of radiation images with varying the doses of radiation that have been read, as well as the doses of radiation applied to record the respective radiation images and which have been detected.
Preferably, the plurality of radiation images are recorded in the at least one radiation recording medium by split recording. Preferably, the plurality of radiation images are recorded in the at least one radiation recording medium using at least one filter having a known radiation transmittance.
In the preferred embodiment, the method further comprises the steps of correcting for the linearity in accordance with a result of the linearity calculation by the computing device. Preferably, a difference between the dose of radiation in the plurality of the radiation images is in a range of from 1:3 to 1:1000 in terms of a ratio. Preferably, when image data of the radiation image is antilogarithm data reverted to a linear state, the linearity is expressed as a ratio in the image data divided a ratio in exposed dose and if the calculated linearity is outside a range of from 0.8 to 1.2 the linearity is corrected.
Preferably, when image data Q0 and Q1 of the radiation images are log converted density data having a 4-digit dynamic range assigned to 10 bits and are read from the radiation images recorded by applying the doses X0 and X1 of radiation, respectively, the linearity is expressed by a following equation and if the calculated linearity is outside a range of from 0.8 to 1.2 the linearity is corrected.
the linearity={(Q1xe2x88x92Q0)/256}/log(X1/X0)
Preferably, the doses of radiation applied to record the radiation images are measured by a dosimeter. Preferably, the at least one radiation recording medium is a stimulable phosphor sheet.